Originally Posted by
yoko1
Possibly, but let me throw out one more thought that relates to both why the procedure was changed and why the crew might not have made the decision you think they would.
...
...
One of the issues that has been highlighted with the original MCAS design is that it operated at a higher speed than the Main Electric Trim in the flaps up configuration. Thus, it is entirely possible that this faster movement could have been interpreted as a problem with the Main Electric Trim, and not one of the automatic sources. Logically, then the crew would attempt to restore the automatic trim thus 1) reintroducing the runaway while 2) taking away the most effective tool to stop it - namely the Main Electric Trim. In fact, this is effectively what happened with ET302. They did not properly run the Runaway Stab procedure, they cutout the trim in a significant out-of-trim conditions, and in an act of desperation they restored the malfunctioning system which promptly drove the stab to the stop.
Note that my proposed 'step 6' (NG or reconfigured MAX switches) is not a memory item, only taken after all electric trim shut down and allows possible restoration of manual electric trim only:
To restore manual electric trim set the [corect switch name] to enabled, be prepared to immediately disable if runaway trim re-occurs. Do not re-enable should this happen.
Could add a "
in event mechanical trim is difficult/impossible " prolog to it.
It does not require crew to determine probable fault cause while performing the memory items, since they remain unchanged.
All of this would be a moot point if mechanical trim as a backup was reliable over the certified range, which it strongly appears not to be.